Beam of yarn sheet monitoring apparatus

ABSTRACT

An optical system for yarn inspecting apparatus to detect broken yarns in a yarn sheet, wherein an air-pressurized tube spans the yarn sheet and blows air in a direction to blow any broken yarn ends through a monitoring axis. The optical system includes a detector head adjacent one edge of the sheet having a light source and a phototransistor and a lens system for projecting light in a narrow cross section beam along the monitoring axis to a retroreflective target at the other edge of the sheet. A target lens covers the target, and the lens and lens system are adjustable to cause the beam to provide a spot of light on the lens and a reduced size light pattern on the target.

PATEN-TED APR 20 L9H SHEET 1 BF 3 PATENTEDAPRZOIQYI 3575.515

SHEET 2 UF 3 \g- L may v a 5' Den 5e Mp T INVENTOR.

EAvMoNoBAmes Fara-n6 CONT/FOL BY 1 CONTflCTS wagm im HTT'OB/VE V6 PATENTEB APR20 |s71 BEAM OF YARN SHEET MONITORING APPARATUS BACKGROUND AND OBJECTS OF THE INVENTION The present invention relates to optical systems for yarn inspection apparatus designed to continuously monitor a large group of yarns arranged to move substantially in unison in side-by'side relation in the form of a warp or warps in one or more feed planes, the group of yarns in each plane being hereinafter referred to as a yarn sheet, to detect occurrence of any broken yarn end in the yarn sheet and produce a defect signal, and more particularly to optical systems for such yarninspecting apparatus adapted to be used with very wide yarn sheets associated with warp-knitting machines and the like, requiring a very long and narrow light beam to span the yarn sheet.

Feeding of yarns in large groups as yarn sheets occurs in many different types of yarn-handling apparatus, such as knitting machines, particularly of the tricot or warp-knitting machine type, in weaving machines, infeeding of yarns from a warping machine to the beam or beams of knitting machines, and similar yarn making and textile-manufacturing installations. When breakage occurs in any of the yarns making up such a yarn sheet, the sudden release of tension on the yarn, its twist characteristics, and the condition of the yarns in such yarn sheets cause the broken yarn end to engage and become entangled or cling or adhere to an adjacent yarn rather than the broken yarn end falling freely out of the plane of the unbroken yarns. Immediate detection of any broken yarn end is essential for a number of reasons, as to avoid costly waste from production of defective fabric by the knitting machine into which the yarns are being fed, and to avoid rapid multiplication of yarn breakage as broken yarns cling to adjoining yarns and exert stresses thereon, which would increase the time required to correct the breakage situation and place the knitting machines back in service. Mechanical stop motions such as have been heretofore used in some types of textile machines having a relatively small number of yarns in each warp are not readily suitable for other types of textile machines, such as tricot knitting machines and like devices having several thousand yarns in each yarn sheet, because of the lack of sufficient space for the yam-sensing elements of such mechanical stop motions and because their effective use usually requires the released portion of the broken yarn to normally move freely beyond the plane of the yarn sheet.

Heretofore, it has been proposed to provide such textileknitting machines with photoelectric broken end detectors, wherein a light beam is directed transversely across the width of the yarn sheet and is spaced slightly to one side of the plane of the yarn sheet, in association with some type of pressurized air tube located adjacent the opposite side of the yarn sheet directing air currents therethrough in a direction to propel any broken yarn ends through the light beam so as to vary the intensity thereof and produce an output signal from the photocell indicating detection of the broken yarn. U.S. Pat. No. 2,438,365 to Hepp and US. Pat. No. 2,711,093 to Edelman are typical of such detectors. Such systems, however, have been subject to considerable problems, due to the long light path involved, the considerable vibration present in such textile machines and the difficulty of making such optical detection systems compatible with this vibration, the difficulty of detecting very fine denier yarn as now frequently used in such textile machines over the long light paths involved, the vulnerability of such systems to respond to spurious signals, because of their sensitivity to electrical surges from stopping and starting of other machines, line voltage fluctuations, and the presence of particles of dust and lint in the region of the detecting system. Also, it is difficult to find appropriate places to locate both the sizable light-transmitting and light-receiving units of said systems in the limited space available in knitting machines,- as the machine parts frequently restrict severely the space where the yarn can be monitored. Additionally, the difficulties of alignment and preservation of alignment of such devices and the high skills required for alignment of such systems when they get out of adjustment, have all posed seri ous practical problems in attempted commercial use of such optical broken yarn detectors.

US. Pat. application Ser. No. 7l7,076, entitled Yarn Inspection Apparatus, filed Mar. 29, I968, by me, jointly with Lawrence Creigh Nickell and Henry T. Sessions, discloses yarn inspection apparatus for monitoring yarn sheets of various types of yarn-processing equipment, including several types of warp-knitting machines, wherein a monitoring light beam spans the width of the yarn sheet and is spaced slightly out of the plane thereof, and wherein the light for the light beam is projected from a detector head adjacent one edge of the yarn sheet and is returned to the detector head by a retroreflective target assembly located adjacent the other edge of the yarn sheet. This arrangement is designed to achieve reliable monitoring of fine yarn in yarn sheets of large width, to minimize vibration and alignment problems, to increase sensitivity of the system, and provide a device wherein the optical components are more readily adaptable to be fitted into the limited spaces available in yarn-handling machines, and which overcomes or minimizes the disadvantages and problems introduced by prior art devices. The retroreflectivetype optical system of the present invention is designed for use with yarn inspection systems of the types disclosed in said earlier application Ser. No. 717,076, and is especially designed for use where a very long and narrow light beam is required because of the presence of a very wide yarn sheet, such as when using the yarn inspection system of said application with a 252-inch warp-knitting machine. The optical system of the pre's'ent invention is designed to increase the amount of light being reflected back to the detector head by the retroreflective target assembly, and to provide sufficient surface area at the target so that a reasonable amount of movement of the light beam such as may be caused by vibration of the knitting machine can be tolerated.

An object of the present invention, therefore, is the provision of j a novel optical system for a detector head and retroreflective target assembly producing a monitoring beam for a broken yarn detector system associated with a yarn sheet, to provide a very long and narrow light beam providing improved toleration of vibration induced by the textileprocessing machine on which it is mounted and to provide high-sensitivity characteristics.

Another object of the present invention is the provision of a novel optical system as described in the immediately preceding paragraph, having a monitoring beam for a very wide yarn sheet, for example, requiring a monitoring beam spanning a distance of more than 10 feet and even up to at least about 22 feet.

Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.

SUMMARY OF THE INVENTION The optical system of the present invention comprises a detector head adjacent an edge of a yarn sheet to be monitored, having a lamp,- a lens system, and a semitransparent mirror through which light from the lamp is transmitted to the lens system to form a narrow beam directed transversely across and adjacent the yarn sheet A target assembly is disposed adjacent the other edge of the yarn sheet, and comprises a sheet or surface of retroreflective material enshrouded by a lens barrel structure supporting a target assembly lens spaced from and covering the retroreflective material in the direction of the detector head. The detector head lens system produces images of the lamp filament on the target assembly lens wherein the filament image size is only a fraction, for example, about half, of the diameter of the target lens and the target lens focus is adjustable to reduce the filament image size on the retroreflective material anywhere from a small spot to a spot slightly smaller than the filament image on the target lens.

The light retrorefiected by the target material is returned through the detector head lens system to the semitransparent mirror, where it is reflected to a phototransistor in the detector head.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a fragmentary, somewhat diagrammatic perspective view of a typical installation of the broken yarn detecting apparatus optical system of the present invention in conjunction with a three-bar triple-width warp-knitting machine, illustrating the locations of the detector heads and targets and the monitoring beams produced thereby;

FIG. 2 is a somewhat diagrammatic, fragmentary vertical section view of the three-bar knitting machine and detecting apparatus shown in FIG. 1, taken along the line 2-2 of FIG. 1;

FIG. 3 is a diagrammatic view of the optical system of the present invention, with the detector head and the target assembly shown in vertical section; and

FIG. 4 is a block diagram of the complete retroreflective detection system.

DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT Referring to the drawings, wherein like reference characters designate corresponding parts throughout the several FIGS., and particularly to FIGS. I and 2 diagrammatically illustrating the type of broken-yam-detecting system and associated warp-knitting machine with which the optical system of the present invention is to be used, there are shown certain components of a conventional wide-warp-knitting machine, including an upper warp lwam ll a middle warp beam 12, and a lower warp beam 13. The yarns are fed from these warp beams under tension in yarn sheets 14, and 16, through the conventional reeds 17a, 17b and 170, and tension bars 18a, 18b and 18c, to the needle position indicated generally at 19 where the yarns are knitted to form the knitted cloth product. The knitting machine illustrated schematically in FIGS. 1 and 2 is, for example, a triple-width machine providing a yarn sheet approximately 252 inches wide, although it will be appreciated that the installation will be similar for a double-width machine producing a yarn sheet of about 168 inches or more width, and that the system of the present invention can be used if desired in a single-width knitting machine having the usual yarn sheet width of about 84 inches.

Disposed in the zone between the tension bars 18a, 18]) and 18c and the needle zone 19, and to the rear of the rearmost yarn sheet 16 fed from the lower beam 13, as viewed in FIG. 1, are air discharge tubes 20, three tubes 20a, 20b and 200 being shown disposed in axial alignment so as to substantially span the width of the yarn sheets. Each of the air tubes have an elongated air discharge slit or a plurality of spaced discharge apertures extending or spaced substantially along the length thereof and facing generally toward the yarn sheets. The air discharge tubes 20 are supplied with pressurized air through suitable tubing generally indicated at 21, from a conventional motor driven blower 22.

Monitoring beam assemblies 24, and 26 associated respectively with the yarn sheets 14, 15 and 16 are located on the opposite side of the yarn sheets l4, l5 and 16, relative to the location of the air discharge tubes 20, and disposed slightly upstream therefrom relative to the direction of travel of the yarn, to detect any broken yarn ends propelled from the planes of the associated yarn sheets in the direction of the monitoring beams assigned thereto by the air discharged from the air discharge tubes 20. It will be appreciated that some latitude is permissible in regard to the number of monitoring beam assemblies used in relation to the number of yarn sheets. For example, as disclosed in said earlier US. Pat. application Ser. No. 7l7,076, a warp-knitting machine having two warp beams producing only two yarn sheets may be monitored by two monitoring beam assemblies respectively associated with the two yarn sheets, or in the case of a double-width knitting machine having two warp beams, three monitoring beam assemblies may be employed, one for the front yarn sheet spanning the entire width thereof, and two for the rear yarn sheet, each spanning one-half of the yarn sheet. Also, in the case of a triple-width warp-knitting machine, if only two warp beams producing two yarn sheets are present, only two monitoring beam assemblies may be used, respectively associated with the two yarn sheets, or, if desired, a single monitoring beam assembly spanning the entire width and located in front of the front yarn sheet may be used, relying upon the discharge air from the air tubes to blow broken yarn ends from either the front yarn sheet or the rear yarn sheet into the path of the monitoring beam to produce detection of a broken end. Similarly, if a triple-width warp-knitting machine having three warp beams is to be provided with the broken end detector system, either a pair of monitoring beam assemblies, one located in front of the front yarn sheet, and the second located between the front yarn sheet and the intermediate yarn sheet, may be employed similar to the arrangement shown in FIG. 2 of said earlier patent application, or three monitoring beam assemblies respectively associated with the three yarn sheets may be employed as illustrated in FIG. 1 of this application and indicated by the reference characters 24, 25 and 26.

Each of the monitoring beam assemblies, 24, 25 and 26 are identical in construction and a description of the details of one will suffice for each. Referring, for example, to the monitoring beam assembly 24 disposed in front of the front yarn sheet 14, the assembly comprises a detector head 28 fixedly mounted in any suitable manner, for example, by shock mounts outwardly of one edge of the associated yarn sheet, such as the yarn sheet 14, for directing a high intensity light beam 29 of narrow cross section along a rectilinear axis so as to span the entire width of the yarn sheet and be spaced slightly from the plane of the associated yarn sheet to the side thereof opposite the side on which the air discharge tubes 20 are located. The detector head 28 is of very similar construction to that illustrated and described in said earlier application, and comprises an outer casing 30 of generally box-shaped configuration having an apertured front end wall 31 provided with an opening 31a forming an optical aperture. A mounting block 32, here formed of two assembled drilled block members, is disposed within the casing and carried by the front end wall 31, to rigidly support in proper alignment the lamp 33, phototransistor 34, semitransparent mirror 35 and biconcave lens 36 in proper alignment. The mounting block 32 in the illustrated embodiment includes a first bore section 3211 communicating with the biconcave lens 36 and interrupted by the semitransparent mirror 35 in its intermediate region, and communicating at its end opposite the biconcave lens 36 with a constricted bore section 32b terminating in a small aperture 32b through which the light from the lamp 33 passes into the bores 32b and 32a. A branch bore section 32c extends at right angles to the bore section 32a along the reflected light path of the mirror 35 for return light entering through the biconcave lens 36 into the bore section 32a to permit passage of this reflected light to the photosensitive surface of the phototransistor 34.

The lamp 33 is preferably a GE-type I974 quartz iodine lamp, which has a long and constant life, high light intensity, and has a small filament which makes it well suited for this application. This lamp is supported in an appropriate bore in a portion of the mounting block 32 and may be retained therein, for example, by a setscrew 33a to dispose the filament in alignment with the small aperture 32b at the rearmost end of the bore section 32b. Projecting externally from the front end wall 31 of the casing in concentric relation with the projected axis of the opening 31a is a lens tube mounting block 37 having a bore for slidably receiving a portion of a lens tube 38 therein carrying a planoconvex lens 39. The lens tube 38 may be adjusted axially relative to the lens tube mounting block 37 in any of several known ways, such as by a focusing adjustment screw 38a as shown in FIG. 2 which extends through the enlarged outer portion of the lens tube 38 and is threaded into a threaded opening in the lens tube mounting block 37.

In the preferred example, the biconcave lens 5b has a focal length of l2.5 millimeters and the planoconvex lens has a focal length of 75 millimeters. The light from the lamp 33 passes through the small aperture 321) and the semitransparent mirror 35 to the biconcave lens 1%. A beam of light is formed by this lens 36, which acts as though it came from a much smaller light source. The light from the lens 3% is collected by the planoconvex lens 39 and is directed at the beam 29 toward the target assembly generally indicated at 40.

As illustrated in H6. 3, the retroreflective target assembly of the present invention comprises a mounting plate 41 having apertures permitting it to be secured to a suitable stationary support, such as the T-shaped support 410 illustrated in H6. 1, in appropriate alignment with its associated detector head. Positioned on the mounting plate is a target member 42 in the form of a panel or sheet or retroreflective material, such, for example, as retroreflective tape manufactured under the trade name Scotchlite, by Minnesota Mining and Manufacturing Company, having the property of returning light along the same path as the incident light rays regardless of the angle of incidence. The retroreflective target member 42 is surrounded by a lens tube mounting block 43 fixed to the mounting plate 41, as by screws 43', in which is slidably supported a lens tube 44 secured in desired position by a locking screw and sup porting a target assembly lens 45, which in the illustrated embodiment is a 50 millimeter focal length planoconvex lens having a diameter of 48 millimeters.

The light from the lamp 33 of the detector head 28 which is passed through the semitransparent member 35, formed into a beam by the l2.5 millimeter focal length double-concave lens 36 and collected by the planoconvex 75 millimeter focal length lens 39, is substantially focused on the target assembly lens 45. For the smallest beam size, the lens 39 in the detector head should be substantially focused so that the image of the lamp filament is projected on the center of the target assembly lens 45. The size of the filament image on the face of the lens 45 at a distance of 22 feet from the detector head 28, which is the exemplary distance for the monitoring beam to be used with a 252-inch warp-knitting machine, is about 1 inch by three-quarters of an inch, which allows considerable room for movement of the image. The focus of the target assembly lens 45 is adjustable by movement of the lens tube 44 in the lens tube mounting block 43, and can reduce the image of the filament at the target member 42 anywhere from an extremely small spot to a spot slightly smaller than the filament image on the lens 45. This enables the sensitivity of the system to be varied over wide limits with the larger spot producing much less sensitivity but greatly improving the ability of the system to withstand movements of the light beam. By making the target assembly lens 45 adjustable, an optimum signal to noise ratio can be found for given installation, taking into account both light and electrically caused noise signals.

After the spot of light from the beam 29 is formed on the retroreflective tape or material 42 by the target assembly lens 45, it is reflected back through the lens 45 along the beam axis to the detector head 28, passing through the lenses 39 and 36 and being reflected by the semitransparent mirror 35 to the phototransistor 34. The two lenses 39 and 36 in the detector head 27 reduce the beam of light returning from the target assembly to a small intense spot, which is then reflected to the phototransistor 34.

While the lens in the above-described example is indicated as a 50 millimeter focal length lens, the focal length of this lens is not too critical and the 50 millimeter size chosen was governed more by space considerations rather than optimum performance. Space around knitting machine is rather limited and the lens chosen represent a good compromise between performance and space considerations. However, it was found that a longer focal length lens, 100 millimeters, for example, will give much greater sensitivity while a smaller focal length lens will give less sensitivity and is more critical to adjust. Biconvex lenses of similar plarioconvex length give comparable results to those achieved with the planoconvex lens.

in the preferred example, the filament of the lamp 33 and the small aperture 32b in the detector head block 32 are displaced downwardly about one thirty-second inch from the axis of the lens system, to take care of refraction occurring in the semitransparent member 35.

The lens tube 44 of the target assembly 40 not only provides for focusing adjustment of the target assembly lens 45, but also serves as a protective device for the lens 45, protecting it from physical damage and to a certain extent from dust. The lens tube 44 and lens tube mounting block 43 also tend to block ambient light from the retroreflective surface 42, although in most cases this is not an important factor. It has been found that turning off and on the lights in a normal room produces less signal than the dust particles in the air between the detector head and the target assembly, and the directional sensitivity of the target assembly is such that a flashlight beam on the target assembly does not produce a detectable reflected beam unless it is almost directly on the axis of the lens 45.

Another advantage of the optical system above described is the protection of the retroreflective material 42 from oil, dust and other contaminants which seriously degrade the reflective properties of the tape. Obviously, it is much easier to clean the exposed surface of the lens 45 than it would be to clean the surface of the retroreflective material. While a piece of flat glass was used for protection of the tape in the system disclosed in said earlier US. Pat. application Ser. No. 717,076, this resulted in a decrease of sensitivity of about 16 percent and presented reflection problems. On the other hand, the use of the lens 45 in front of the retroreflective surface greatly increases the sensitivity of the system and eliminates the reflection problems.

Although the system disclosed is illustrated as covering a distance of 22 feet, obviously it can be used for both longer and shorter distances. If the beam size needs to be smaller, the lens in the detector and the size of the limiting aperture in the detector head can be changed accordingly. A shorter focal length concave lens or a longer focal length convex lens or both can be used to accomplish this.

it will be apparent that the use of the retroreflective material 42 in conjunction with the target assembly lens 45 greatly reduces the problems which would otherwise arise from movement of the light beam, such as when vibration of the machine transmitted to the detector head produces translation of the beam. ln a receiver-type photoelectric control system, wherein the beam transmitted from a light transmitter is received in a separate receiver component at the opposite edge of the yarn sheet, such receivers using modern solid-state detectors such as phototransistors and a lens system would be very sensitive to any movement of the light beam. This is because the sensitive area of phototransistors is usually about one-sixteenth of an inch or less, and any appreciable movement of the light beam, either with or without a lens system associated with the receiver, would produce a large change in the output signal. This is not true with the retroreflective system of the present invention, since the focus light spot would simply move to another place on the retroreflective tape.

It will be observed from FIG. 3 that the detector head 28 includes a preamplifier circuit, indicated at M, which may be the preamplifier schematic circuit shown in FIG. 9 of said earlier application Ser. No. 717,076, and described in connection therewith, which consists essentially of an emitter follower presenting a high impedance to the calibration resistor designated 42-R2 in said earlier application, in series with the phototransistor, and to present a low impedance to the main amplifier of the system which permits use of a long connecting cable if required. P16. 4 of the present application shows a block diagram of the complete retroreflective detection system, including the detector head 25, the target assembly 40, a main amplifier 49 connected to the detector head to receive signals from the preamplifier as, a relay driver 50, and a relay 51 which can be used for alarm or control purposes. The specific electronic circuit employed with the optical system of the present invention may be those described in detail and illustrated in said earlier application Ser. No. 717,076.

lclaim:

1. ln yarn inspection apparatus for detecting broken yarns in a yam sheet lying in a feed plane by detection of broken yarn ends moving through a selected inspection axis extending transversely of the yarn sheet and spaced to one side of the feed plane in a direction perpendicular thereto, the apparatus including air pressurized rectilinear elongated tube means located to the opposite side of said feed plane from said inspection axis in parallelism therewith and transversely spanning the yarn sheet for discharging air toward the yarns to blow any broken yarn ends through said axis; an optical system producing a monitoring light beam along said inspection axis and spanning the yarn sheet comprising a detector head located outwardly adjacent one edge of the yarn sheet; said detector head including a lamp having a filament of selected size for producing light, a semitransparent mirror, and a lens system located along a first optical axis for projecting light from said lamp passing through said mirror and lens system in a first direction in a narrow cross section monitoring beam along said inspection axis, said detector head including a phototransistor located along a branch optical axis and intersecting the same at said mirror to receive light passing through said lens system toward said mirror in a direction opposite said first direction; a retroreflective target assembly located outwardly adjacent the opposite edge of the yearn sheet along said inspection axis including a substantially fiat surface of retroreflective material of selected area facing the incident rays projected from said detector head lens system for retroreflecting incident rays of said beam striking said retrorefiective material back along their incident ray paths, and a target lens in the path of incident rays of the beam projected by the detector head lens system and disposed in covering relation to said retroreflective surface, each of said detector head lens system and said target lens being adjustable along said inspection axis to provide a spot of light on the target lens having a maximum dimension which is only a fraction of the diameter of the target lens and the target lens being adjustable to reduce said spot of light to a smaller size light pattern on said retroreflective material.

2. In yarn inspection apparatus an optical system as defined in claim 1, wherein said target assembly includes a mounting plate having a flat surface portion, said retroreflective material being supported on said flat surface portion, a lens tube for supporting said target lens, and an annular mount for movably supporting said lens tube and enshrouding said retrorefiective material.

3. ln yarn inspection apparatus, an optical system as defined in claim 1, wherein said detector head lens system comprises a biconcave lens disposed along said first optical axis for receiving light from said lamp transmitted through said semitransparent mirror and a planoconvex lens spaced along said first optical axis from said biconcave lens toward said target assembly for collecting light from the biconcave lens and focusing the same on said target lens.

4. In yarn inspection apparatus, an optical system as defined in claim 2, wherein said detector head lens system comprises a biconcave lens disposed along said first optical axis for receiving light from said lamp transmitted through said semitransparent mirror and a planoconvex lens spaced along said first optical axis from said biconcave lens toward said target assembly for collecting light from the biconcave lens and focusing the same on said target lens.

5. ln yarn inspection apparatus, an optical system as defined in claim I, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.

6. ln yarn inspection apparatus, an optical system as defined in claim 2, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.

7. ln yarn inspection apparatus, an optical system as defined in claim 3, wherein said target lens is a planoconvex lens.

8. ln yarn inspection apparatus, an optical system as defined in claim 3, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.

9. ln yarn inspection apparatus, an optical system as defined in claim 3, wherein said biconcave lens has a focal length of about l2.5 millimeters and said planoconvex lens of said detector head lens system has a focal length of about 75 millimeters.

10. In yarn inspection apparatus, an optical system as defined in claim 5, wherein said target lens has a focal length of about 50 millimeters.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3,575, 515 Dated April 20, 1971 Inventor(s) Raymond Baines Fertig It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the front page, beside data element number "[54]" and at column 1, line 1, before "BEAM" there should be inserted OPTICAL SYSTEM FOR MONITORING-- Signed and sealed this 6th day of July 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, Attesting Officer Commissioner of Pate 

1. In yarn inspection apparatus for detecting broken yarns in a yarn sheet lying in a feed plane by detection of broken yarn ends moving through a selected inspection axis extending transversely of the yarn sheet and spaced to one side of the feed plane in a direction perpendicular thereto, the apparatus including air pressurized rectilinear elongated tube Means located to the opposite side of said feed plane from said inspection axis in parallelism therewith and transversely spanning the yarn sheet for discharging air toward the yarns to blow any broken yarn ends through said axis; an optical system producing a monitoring light beam along said inspection axis and spanning the yarn sheet comprising a detector head located outwardly adjacent one edge of the yarn sheet; said detector head including a lamp having a filament of selected size for producing light, a semitransparent mirror, and a lens system located along a first optical axis for projecting light from said lamp passing through said mirror and lens system in a first direction in a narrow cross section monitoring beam along said inspection axis, said detector head including a phototransistor located along a branch optical axis and intersecting the same at said mirror to receive light passing through said lens system toward said mirror in a direction opposite said first direction; a retroreflective target assembly located outwardly adjacent the opposite edge of the yearn sheet along said inspection axis including a substantially flat surface of retroreflective material of selected area facing the incident rays projected from said detector head lens system for retroreflecting incident rays of said beam striking said retroreflective material back along their incident ray paths, and a target lens in the path of incident rays of the beam projected by the detector head lens system and disposed in covering relation to said retroreflective surface, each of said detector head lens system and said target lens being adjustable along said inspection axis to provide a spot of light on the target lens having a maximum dimension which is only a fraction of the diameter of the target lens and the target lens being adjustable to reduce said spot of light to a smaller size light pattern on said retroreflective material.
 2. In yarn inspection apparatus an optical system as defined in claim 1, wherein said target assembly includes a mounting plate having a flat surface portion, said retroreflective material being supported on said flat surface portion, a lens tube for supporting said target lens, and an annular mount for movably supporting said lens tube and enshrouding said retroreflective material.
 3. In yarn inspection apparatus, an optical system as defined in claim 1, wherein said detector head lens system comprises a biconcave lens disposed along said first optical axis for receiving light from said lamp transmitted through said semitransparent mirror and a planoconvex lens spaced along said first optical axis from said biconcave lens toward said target assembly for collecting light from the biconcave lens and focusing the same on said target lens.
 4. In yarn inspection apparatus, an optical system as defined in claim 2, wherein said detector head lens system comprises a biconcave lens disposed along said first optical axis for receiving light from said lamp transmitted through said semitransparent mirror and a planoconvex lens spaced along said first optical axis from said biconcave lens toward said target assembly for collecting light from the biconcave lens and focusing the same on said target lens.
 5. In yarn inspection apparatus, an optical system as defined in claim 1, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.
 6. In yarn inspection apparatus, an optical system as defined in claim 2, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.
 7. In yarn inspection apparatus, an optical system as defined in claim 3, wherein said target lens is a planoconvex lens.
 8. In yarn inspection apparatus, an optical system as defined in claim 3, wherein said target lens is a planoconvex lens having a diameter comparable to its focal length.
 9. In yarn inspection apparatus, an optical system as defined in claim 3, wherein said biconcave lens has a focal length of about -12.5 millimeters and said planoconvex lens of said detector head lens system has a focal length of about 75 millimeters.
 10. In yarn inspection apparatus, an optical system as defined in claim 5, wherein said target lens has a focal length of about 50 millimeters. 